Fern and lycophyte guard cells do not respond to endogenous abscisic acid.

Stomatal guard cells regulate plant photosynthesis and transpiration. Central to the control of seed plant stomatal movement is the phytohormone abscisic acid (ABA); however, differences in the sensitivity of guard cells to this ubiquitous chemical have been reported across land plant lineages. Using a phylogenetic approach to investigate guard cell control, we examined the diversity of stomatal responses to endogenous ABA and leaf water potential during water stress. We show that although all species respond similarly to leaf water deficit in terms of enhanced levels of ABA and closed stomata, the function of fern and lycophyte stomata diverged strongly from seed plant species upon rehydration. When instantaneously rehydrated from a water-stressed state, fern and lycophyte stomata rapidly reopened to predrought levels despite the high levels of endogenous ABA in the leaf. In seed plants under the same conditions, high levels of ABA in the leaf prevented rapid reopening of stomata. We conclude that endogenous ABA synthesized by ferns and lycophytes plays little role in the regulation of transpiration, with stomata passively responsive to leaf water potential. These results support a gradualistic model of stomatal control evolution, offering opportunities for molecular and guard cell biochemical studies to gain further insights into stomatal control.